Such wind turbines usually have rotors designed as propellers with two or more blades. The power delivered by the rotor-driven generator is a function of wind velocity, depending on the profile of the propeller blades as well as on their pitch which can be adjusted by a variety of means (see, for example, U.S. Pat. Nos. 4,348,155 and 4,352,634). The rotor shaft is customarily mounted horizontal on a vertical mast which in turn can rotate to let the propeller, acting as a weather vane, find a position in a plane generally perpendicular to the prevailing wind direction.
A problem in such wind turbines is the danger of undue acceleration in the event of a gale. Protection against excessive wind velocities can be had by a feathering of the propeller blades or, if their pitch is not variable, by the extension of baffles or the application of a braking force.
According to the laws of aerodynamics, an air current has a tendency of detaching itself from a propeller surface--a condition referred to hereinafter as break-off--when the air speed along the blade surfaces becomes incompatible with laminar flow on account of excessive relative acceleration or retardation of the rotor. The art, however, appears to have been reluctant to apply this principle to a wind turbine, especially one of the single-blade type, for fear that such a break-off might give rise to the annoying phenomenon known as stall flutter.